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Stromatolites (from Greek στρώμα, strōma, mattress, bed, stratum, and λιθος, lithos, rock) are layered accretionarystructures formed in shallow water by the trapping, binding and cementation of sedimentary grains by biofilms of microorganisms, especially cyanobacteria (commonly known as blue-green algae). They include some of the most ancient records of life on Earth.

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Morphology

A variety of stromatolite morphologies exist including conical, stratiform, branching, domal,[1] and columnar types. Stromatolites occur widely in the fossil record of the Precambrian, but are rare today. Very few ancient stromatolites contain fossilizedmicrobes. While features of some stromatolites are suggestive of biological activity, others possess features that are more consistent with abiotic (non-organic) precipitation. Finding reliable ways to distinguish between biologically-formed and abiotic (non-biological) stromatolites is an active area of research in geology.

Stromatolites are a major constituent of the fossil record for about the first 3.5 billion years of life on earth,[4] with their abundance peaking about 1,250 million years ago. They subsequently declined in abundance and diversity, which by the start of the Cambrian had fallen to 20% of their peak. The most widely-supported explanation is that stromatolite builders fell victims to grazing creatures (the Cambrian substrate revolution), implying that sufficiently complex organisms were common over 1 billion years ago.[5][6][7]

The connection between grazer and stromatolite abundance is well documented in the younger Ordovicianevolutionary radiation; stromatolite abundance also increased after the end-Ordovician and end-Permian extinctions decimated marine animals, falling back to earlier levels as marine animals recovered.[8]

Cyanobacteria use water, carbon dioxide, and sunlight to create their food. The byproducts of this process are oxygen and calcium carbonate (lime). A layer of mucus often forms over mats of cyanobacterial cells. In modern microbial mats, debris from the surrounding habitat can become trapped within the mucus, which can be cemented together by the calcium carbonate to grow thin laminations of limestone. These laminations can accrete over time, resulting in the banded pattern common to stromatolites. The domal morphology of biological stromatolites is the result of the vertical growth necessary for the continued infiltration of sunlight to the organisms for photosynthesis.

^Allwood, Abigail; Grotzinger, Knoll, Burch, Anderson, Coleman, and Kanik (2009). "Controls on development and diversity of Early Archean stromatolites". Proceedings of the National Academy of Sciences.